101 research outputs found
Strange New Worlds: Is Earth Special?
Phil Plait is an American astronomer, skeptic, writer and brainchild behind the popular science blog âBad Astronomy,â on which he tries to debunk scientific myths and misconceptions.
Though never a NASA employee, he has worked as part of the Hubble Space Telescope team as well as engaging in public outreach advocacy for several NASA missions focused on high-energy forms of light emitted by black holes, exploding stars and super-dense neutrons stars. Plait is the author of two books, Bad Astronomy and Death from the Skies!, in which he provides real science behind all the ways astronomical events could wipe out life on Earth. Plait attended the University of Michigan and earned a Ph.D. in astronomy from the University of Virginia
Space Telescope Imaging Spectrograph Parallel Observations of the Planetary Nebula M94-20
The planetary nebula M94--20 in the Large Magellanic Cloud was
serendipitously observed with the Space Telescope Imaging Spectrograph on board
the Hubble Space Telescope as part of the Hubble Space Telescope Archival Pure
Parallel Program. We present spatially resolved imaging and spectral data of
the nebula and compare them with ground based data, including detection of
several emission lines from the nebula and the detection of the central star.
We find the total H alpha + [NII] flux = 7.3e-15 erg s^-1 cm^-2 and we estimate
the magnitude of the central star to be m_V = 26.0 +/- 0.2. Many other H alpha
sources have been found in M31, M33 and NGC 205 as well. We discuss the use of
the parallel observations as a versatile tool for planetary nebula surveys and
for other fields of astronomical research.Comment: Latex, 14 pages, 2 JPEG figures, 2 tables. PASP Research Note, June
1999, in pres
A Candidate Substellar Companion to HR 7329
We present the discovery of a candidate substellar companion from a survey of
nearby, young stars with the NICMOS coronagraph on the Hubble Space Telescope.
The H ~ 12 mag object was discovered approximately 4" from the young A0V star
HR 7329. Using follow-up spectroscopy from STIS, we derive a spectral type
between M7V and M8V with an effective temperature of ~ 2600 K. We estimate that
the probability of a chance alignment with a foreground dwarf star of this
nature is ~ 10^(-8) and therefore suggest the object (HR 7329B) is physically
associated with HR 7329 with a projected separation of 200 AU. Current brown
dwarf cooling models indicate a mass of less than 50 Jupiter masses for HR
7329B based on age estimates of < 30 Myr for HR7329A.Comment: 8 pages LATEX, 5 ps figures, accepted for Ap
The Late-time Expansion of the Ejecta of SN 1987A
The evolution of the shape and size of the ejecta of SN 1987A is analyzed
over a period of ~ 8 years based on HST images and spectra taken between 1278
and 4336 days after the supernova outburst. We combine both proprietary and
archival HST data obtained with the FOC, WFPC2 and STIS. The low resolution
near-UV prism FOC spectrum obtained at day 3043 has not been described
previously. Although the FWHM of the ejecta grew linearly over the time span
studied, the appearance of the SN envelope also changed markedly with
wavelength. At visible wavelengths (lambda ~ 5000 Angstrom) the ejecta became
progressively more elongated, reaching an ellipticity epsilon ~ 0.25 by day
4000. In the near-UV (lambda ~ 2500 AA), the ejecta remained closely circular
(epsilon <= 0.1) and ~ 50% larger in angular extent than in the visible. The
FOC prism observations show that the large extent of the SN envelope is
confined to a grouping of resonance lines spanning Mg I 2852, Mg II 2795,2802
and several Fe II multiplets -- thereby confirming that the larger size of the
debris in the near-UV is due to scattering in these optically thick transitions
compared to the optically thin forbidden and semi-forbidden transitions that
dominate the visible spectrum. The available data are not of sufficient quality
to detect the slight deviation from linear expansion expected for the outermost
regions of the near-UV images as predicted by Chugai et al. (1997).Comment: Accepted for publication in A&A. 8 pages with 5 figures. Latex using
Springer's A&A macros v4.0
Reverse sequence polymerization-induced self-assembly in aqueous media: a counter-intuitive approach to sterically-stabilized diblock copolymer nano-objects
Polymerization-induced self-assembly (PISA) is a powerful platform technology for the efficient synthesis of block copolymer nanoparticles in many types of solvents, including water. In PISA, a soluble precursor block is used to grow a second insoluble block, which leads to in situ self-assembly of the block copolymer chains. Thus, in the case of aqueous PISA, the water-soluble block is always prepared first because this confers steric stabilization. Herein, we challenge this paradigm by demonstrating that amphiphilic diblock copolymer chains can be prepared in water by preparing the hydrophobic block first via reversible additionâfragmentation chain transfer (RAFT) polymerization. This counter-intuitive reverse sequence PISA formulation utilizes an ionic RAFT agent to conduct the RAFT aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA), which results in the formation of charge-stabilized PHPMA latex particles of âŒ500 nm diameter. Initial attempts to chain-extend these hydrophobic PHPMA chains with water-miscible monomers such as glycerol monomethacrylate (GMA) were unsuccessful, with only uncontrolled free radical polymerization being observed in the aqueous phase. However, using a water-immiscible monomer such as isopropylideneglycerol methacrylate (IPGMA) enabled the synthesis of charge-stabilized PHPMA-PIPGMA latex particles. Subsequent acid hydrolysis of the PIPGMA block led to the in situ formation of sterically-stabilized PHPMA-PGMA diblock copolymer spheres. Alternatively, dissolution of the precursor PHPMA latex in a methanol/water binary mixture enables RAFT solution polymerization of water-miscible monomers such as GMA or N,NâČ-dimethylacrylamide (DMAC) to be achieved with good control. The resulting amphiphilic diblock copolymer chains then undergo self-assembly in aqueous solution after removal of the methanol co-solvent. Finally, this reverse sequence PISA protocol can also be applied to other vinyl monomers such as 2-methoxyethyl methacrylate (MOEMA) or diacetone acrylamide (DAAM), which significantly broadens its scope
An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 um) and the coronagraph on the
Near-InfraRed Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space
Telescope (HST) to survey 45 single stars with a median age of 0.15 Gyr, an
average distance of 30 pc, and an average H-magnitude of 7 mag. For the median
age we were capable of detecting a 30 M_Jup companion at separations between 15
and 200 AU. A 5 M_Jup object could have been detected at 30 AU around 36% of
our primaries. For several of our targets that were less than 30 Myr old, the
lower mass limit was as low as a Jupiter mass, well into the high mass planet
region. Results of the entire survey include the proper motion verification of
five low-mass stellar companions, two brown dwarfs (HR7329B and TWA5B) and one
possible brown dwarf binary (Gl 577B/C).Comment: 11 figures, accepted by A
Spatially resolved spectroscopy of planetary nebulae and their halos I. Five galactic disk objects
Strong mass loss off stars at the tip of the asymptotic giant branch (AGB)
profoundly affects properties of these stars and their surroundings, including
the subsequent planetary nebula (PN) stage. With this study we wanted to
determine physical properties of mass loss by studying weakly emitting halos,
focusing on objects in the galactic disk. Halos surround the, up to several
thousand times, brighter central regions of PNe. Young halos, specifically,
still contain information of the preceeding final mass loss stage on the AGB.
In the observations we used the method of integral field spectroscopy with the
PMAS instrument. This is the first committed study of halos of PNe that uses
this technique. We improved our data analysis by a number of steps. In a study
of the influence of scattered light we found that a moderate fraction of
intensities in the inner halo originate in adjacent regions. As we combine line
intensities of distant wavelengths, and because radial intensity gradients are
steep, we corrected for effects of differential atmospheric refraction. In
order to increase the signal-to-noise of weak emission lines we introduced a
dedicated method to bin spectra of individual spatial elements. We also
developed a general technique to subtract telluric lines - without using
separate sky exposures. By these steps we avoided introducing errors of several
thousand Kelvin to our temperature measurements in the halo. For IC3568 we
detected a halo. For M2-2 we found a halo radius that is 2.5 times larger...
(abridged)Comment: 27 pages, 29 figures, A&A (in press), Abridged abstract, Corrected
and clarified various minor issues; the section on scattered light is
significantly clarifie
Imaging and spectroscopy of arcs around the most luminous X-ray cluster RX J1347.5-1145
The cluster RX J1347.5-1145, the most luminous cluster in the X-ray
wavelengths, was imaged with the newly installed Space Telescope Imaging
Spectrograph (STIS) on-board HST. Its relatively high redshift (0.451) and
luminosity indicate that this is one of the most massive of all known clusters.
The STIS images unambiguously show several arcs in the cluster. The largest two
arcs (> 5 arcsec in length) are symmetrically situated on opposite sides of the
cluster, at a distance of ~ 35 arcsec from the central galaxy. The STIS images
also show approximately 100 faint galaxies within the radius of the arcs whose
combined luminosity is ~ 4 x 10^11 Lsun. We also present ground-based
spectroscopic observations of the northern arc which show one clear emission
line at 6730 A, which is consistent with an identification as [OII] 3727 A,
implying a redshift of 0.81 for this arc. The southern arc shows a faint
continuum but no emission features. The surface mass within the radius of the
arcs (240 kpc), as derived from the gravitational lensing, is 6.3 x 10^14 Msun.
The resultant mass-to-light ratio of ~1200 is higher than what is seen in many
clusters but smaller than the value recently derived for some `dark' X-ray
clusters (Hattori et al. 1997). The total surface mass derived from the X-ray
flux within the radius of the arcs is ~2.1 - 6.8 x 10^14 Msun, which implies
that the ratio of the gravitational to the X-ray mass is ~1 to 3. The surface
GAS mass within this radius is ~3.5 x 10^13 Msun, which implies that at least
6% of the total mass within this region is baryonic.Comment: 3 figures. Replaced with the final version as appears in the
Astrophysical Journal Letters (Jan 10, 1998 issue). This incorporates some
important revision
Spatially Resolved STIS Spectroscopy of SN 1987A: Evidence for Shock Interaction with Circumstellar Gas
Visual and ultraviolet spatially resolved (~ 0."1) spectra of SN 1987A
obtained on days 3715 and 3743 with the Space Telescope Imaging Spectrograph on
the Hubble Space Telescope show that the high-velocity SN debris is colliding
with circumstellar gas. Very broad Ly-alpha emission with velocities extending
to ~ +/- 20,000 km/s originates inside the inner circumstellar ring and appears
to fill most of the surface area within 0."67 +/- 0."03 (0.14 pc at a distance
of 50 kpc) of the ring's center. The observed Ly-alpha flux from the shocked
ejecta is (1.85 +/- 0.53) 10^{-13} erg/cm2/s and (1.25 +/- 0.51) 10^{-12}
erg/cm2/s after correcting for extinction. A spatially unresolved blue-shifted
emission feature was discovered in H-alpha (and other lines) on the inner ring
at p.a. 31 +/- 8 degree. The H-alpha emission extends to -250 km/s with no
corresponding red-shifted emission. This highly localized interaction appears
to be the initial contact of the supernova blast wave with an inward protrusion
of the inner ring. The broad Ly-alpha emission and the `hot spot' are separate
interaction phemonena associated with the reverse and forward shocks,
respectively. We also find that the size of the inner ring in forbidden lines
of oxygen has a dependence on ionization potential, in agreement with
photoionization models of the ring.Comment: 11 pages (LaTeX, aaspp4.sty), 8 figures, accepted for publication in
the Astrophysical Journal Letters HST Second Servicing Mission special issu
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